The switching frequency varies with an output voltage VOUT in a typical power converter with COT (constant-on-time) control. To keep the switching frequency constant regardless of variation of the output voltage VOUT, PCOT (pseudo-constant-on-time) control is often applied in the power converters.
FIG. 1 illustrates a prior art PCOT power converter 100. The PCOT power converter 100 as illustrated comprises an on-time generating circuit 101, a comparison circuit 102, a logic circuit 103, a switching circuit 104, a ramp compensation circuit 105 and a feedback circuit 106.
In the illustrated PCOT power converter 100 of FIG. 1, the on time TON of a main switch M1 of the switching circuit 104 is determined by the on-time generating circuit 101 as:
                              T          ON                =                                            C              TON                        ×                          R              FREQ                        ×                          V                              REF                ⁢                                                                  ⁢                1                                                          V            IN                                              (        1        )            
The amplitude VSLOPE(PP) of a ramp compensation signal VSLOPE generated by the ramp compensation circuit 105 is:
                              V                      SLOPE            ⁡                          (              pp              )                                      =                                            (                                                V                  IN                                -                                  V                  OUT                                            )                        ×                          T              ON                                                          R              C                        ×                          C              C                                                          (        2        )            
By substituting equation (1) into equation (2), there is:
                              V                      SLOPE            ⁡                          (              pp              )                                      =                                                            (                                                      V                    IN                                    -                                      V                    OUT                                                  )                            ×                              C                TON                            ×                              R                FREQ                            ×                              V                                  REF                  ⁢                                                                          ⁢                  1                                                                                    R                C                            ×                              C                C                            ×                              V                IN                                              =                                                    C                TON                            ×                              R                FREQ                            ×                              V                                  REF                  ⁢                                                                          ⁢                  1                                            ×                              (                                  1                  -                  D                                )                                                                    R                C                            ×                              C                C                                                                        (        3        )            
As can be observed from equation (3), in the PCOT power converter 100 of FIG. 1, the amplitude VSLOPE(PP) of the ramp compensation signal VSLOPE is partly determined by the duty cycle D of the PCOT power converter 100. In more detail, the larger the duty cycle D is, the smaller the amplitude VSLOPE(PP) is. Such small amplitude of the ramp compensation signal may easily result in an unstable system.